I agree that it's very hard to believe a std older home as the OP
described would have 60% humidity with outside temps of 0 during the
winter with forced air heat. Some small amount of condensation around
windows is normal. I'd check the windows, as leaking drafts, etc
will cause condensation. Further proof is that the dehumidifier
didn't remove much water.

BTW, if you think a heat exchanger is too expensive, how expensive do
you think it would be to implement the idea of just hooking an outside
air intake up to the return air duct? The operating energy wasted
will be huge.

Like just about everyone else, I have the opposite problem and have a
humidifier installed on my forced air furnace. And it does a great
job.